Of your options, I have to say that MaxTheodore strikes me as a particularly great combo -- classic, but with a bright, bold, dynamic feel. Max absolutely works as a standalone, and feels like a natural brother for L and M to me.

I voted Sebastian but I like the Max combo equally as much. Max works on its own and I love its simplicity. Having two kids with an M name wouldn't bother me whatsoever. For me it would take at least 3 to make it a theme.